1. Field of the Invention
The present invention relates to a control system for a continuously variable automatic transmission such as a toroidal type continuously variable automatic transmission and a pulley type continuously variable automatic transmission.
2. Description of Related Art
This kind of automatic transmission control systems controls a rotational speed input to a continuously variable transmission gear mechanism to bring it into agreement with a target rotational speed. Such a continuously variable automatic transmission control system is known from, for example, Japanese Unexamined Patent Publication No. 61-146639.
In gear shift control for the continuously variable automatic transmission which controls the gear ratio so as to bring the engine speed into agreement with a target engine speed determined in accordance with vehicle speed and engine throttle opening, when operating the engine in a zone of low engine speeds and high engine loads to meet a demand for improved fuel efficiency, it is necessary in order to drop the engine speed for the same vehicle speed to perform control to bring the transmission into a high speed transmission ratio (which is equivalent to making the gear ratio lower).
When performing the high speed transmission ratio control, in a state where the automatic transmission is on the side of high speed ratios, the engine operates at a low speed and causes only a small decrease in speed when the engine throttle fully closes in the idle position. For this reason, in the case where a fuel-cut zone is established in order to interrupt fuel delivery to the engine while the engine speed is higher than a specified speed, when the engine throttle fully closes while the automatic transmission is at a high speed ratio, a time for which the engine speed stays in the fuel cut zone is shortened due to a small decrease in engine speed. As a result, it is hard to make the best use of the fuel cut zone, and hence aggravation of fuel efficiency is encountered.